CN1685248A - Apparatus for seismic transmission in subterranean formations and method of doing so - Google Patents

Abstract

The present invention relates to a device for seismic transmission in subterranean formations, comprising one or more transducers of any type, and a method of implementing the same. According to a preferred embodiment, each vibrator comprises a column (1) of at least one sensitive element (for example of the piezoelectric type) between two end plates or slabs (2, 3), and for transferring the vibration signal Signal generator to apply to this bar. The post (1) is coated with a protective sheath (4) and the vibrator is located in a well or cavity (W) and buried in a block of solid coupling material (7) with the sheath (4) and in Two face slabs (2, 3) of at least a part of the upper part of each of their respective faces are in contact, which provide the coupling of the vibrator to the surrounding formation. Several such vibrators can be buried in a well spaced apart from each other. With the chosen delay, the sequential firing of the transducers allows forcing the waves emitted by the device to emit in a preferred direction. Application: eg repeated seismic monitoring during the formation of subsurface reservoirs.

Description

Apparatus for seismic transmission in subterranean formations and method of doing so

field of invention

The present invention relates to an apparatus for seismic transmission in subterranean formations and a method of implementing the same.

Such seismic transmitting equipment finds particular application in the field of seismic work, in which the seismic image of the subterranean formation being investigated is formed by elastic waves picked up by suitable seismic receivers, the subsoil responding to vibrations emitted by sources such as electromechanical vibrators. The waves reflect these waves discontinuously.

The system according to the invention can be used in particular for long-term monitoring operations in subterranean reservoirs (such as liquid reservoirs or oil storage depots) in a developing state, called repeated seismic surveys, in which subsoil obtained at regular intervals are compared layer to detect changes that occur there as a result of its development. Since changes to observations are rather slow, this is a long-lasting operation.

Background Art of the Invention

Seismic monitoring of subsurface reservoirs is typically accomplished by coupling subsurface sources and receivers in various combinations where the sources and/or receivers are arrayed on or near in one or more wells in the overgrown rock formation. A series of seismic transmit-receive cycles are performed according to a technique known as the walk-away technique by each changing the position of the source with respect to the axis of the well where the receiver is fitted, and by recording as a function of travel time t The arrival of receivers R1 to Rn is achieved.

In most cases, the sources used are electromechanical vibrators: electrohydraulic vibrators, piezoelectric vibrators, etc. A vibrator of the piezoelectric type is described, for example, in patent FR-2791180 or in patent US-5360951.

Monitoring of the evolution of the reservoir generally requires seismic monitoring operations at spaced-out intervals. In fact, for each new seismic recording, the surface seismometry equipment must be reinstalled and preferably reproduce the conditions of the launch of the previous seismic operation.

Known methods of monitoring hydrocarbon formations or fluid reservoirs include the use of monitoring systems comprising interconnectors each permanently assembled in shallow wells using connection devices capable of being connected to link cables connected to a seismic laboratory. The receiving antenna formed by the seismic receiver, and the vibrator car that moves within the range.

Especially in the context of periodic monitoring of subterranean reservoirs, the use of movable sources such as vibrators has disadvantages. A movable source does not have sufficient reproducibility in time and space of the emitted seismic waves. It is difficult to locate the source exactly at the same point that it occupied during the previous transmit-receive cycle, and with that exact location, it is difficult to obtain exactly the same ground coupling coefficient.

Patent FR-2728973 (US-5724311) describes a method and an apparatus suitable for permanent seismic monitoring of subterranean formations. In conventional long-term monitoring operations in subterranean areas, the seismic transmitter-receiver equipment is permanently installed on the formation, so that the operating conditions are again stable each time: the same transmitter-receiver position, the same quality of coupling to the formation etc. The apparatus comprises a plurality of seismic sources (eg electromechanical vibrators) at fixed positions on the surface or buried at shallow levels, which are fed or triggered by a central control and recording station. The source and connection network can be buried or permanently mounted on the surface, and the source and connection network are connected to at least one set of receivers permanently coupled to the earth at the surface or to the wall of at least one well passing through the subterranean region . All of these permanently fitted sources coupled to the surrounding rock formations remain stable and at least partially buried this supply network allows a series of Seismic monitoring operations which limit the surface coverage area of this supply network.

Patent FR-2728973 (US-5724311) describes another seismic device suitable for permanent monitoring of subterranean formations by means of one or more seismic transmit-receive groups, each of which includes a source such as a vibrator, and a receiving antenna, It consists of a plurality of elastic wave receivers, such as geophones and/or hydrophones, lowered in the well and coupled to the formation. The source can be placed on the surface on a concrete block fixed to the ground. Preferably fixed to the slab sandstone in the cavity near the well or formed by widening the part of the well in the upper part to reduce disturbances associated with the variation of the measured specific gravity of the bottom. The receiver and source are connected to external signal acquisition and control stations. The operations that allow the placement of these devices are relatively simple and the surface coverage area in various wells is reduced, which facilitates their concentration in the reservoir-forming interval.

By utilizing these permanently mounted sources that are easily focused in reservoir-forming or fluid-storage intervals, and whose coupling qualities to surrounding formations are known and stable, a series of seismic monitoring operations can be performed under similar operating conditions. Realize next. Sets of seismic traces can be compared advantageously, and their differences reflect changes produced in the rock formation.

The previously described vibrator couples to the rock formation through a limited surface, which has significant disadvantages. In fact, the radiation pattern promotes the formation of surface waves as well as S-type waves, which propagate horizontally, disturbing the recording and complicating its processing. Furthermore, their compressional wave generation is relatively low and due to their relatively small burial depth, variations in the petroelastic properties of weathered zones due to climatic conditions cannot be completely ignored by these waves.

Summary of the invention

The device according to the invention is suitable for emitting waves in subterranean formations. It comprises one or more vibrators each comprising two slabs, at least one moving element adapted to generate vibrations and to communicate with the slabs, and for applying a periodic control signal to the moving element on the generator. Characterized in that the or each vibrator is located in the well or cavity and is embedded in at least one solid material providing its coupling with the subterranean formation, this material being associated with two Thick plates are in contact.

Each vibrator can include a fastening rod associated with at least one slab to increase the coupling of the vibrator to the block of coupling material.

According to one embodiment, each slab comprises at least two plates arranged at a distance from each other and connected by fastening bars.

Preferably, the outer surface of each plate and the outer surface of the fastening rod are provided with non-uniform undulations (grooved surfaces) to increase the area over which the device is coupled to the coupling material.

The slot can be perforated to facilitate penetration of the coupling material in the space contained between the two plates.

For example, a one-sided coating may be used to provide coupling to the rock formation at least at opposite end configurations of the vibrator. It is possible to use at least two different coatings, a first material distributed on two separate blocks on their opposite end faces to provide coupling of the vibrator to the formation, and a second material interposed between the two blocks.

Due to the close contact of its thick plates with the coupling material, the efficiency of the vibrator is improved and the emission of S-waves is greatly attenuated by the relative motion of the two plates.

According to a preferred embodiment, the device comprises several transducers connected to a signal generator, the transducers being arranged at intervals relative to each other along the well and all of the transducers being buried in the coupling material. A control box can be inserted between the vibrator and the signal generator, thus allowing them to be triggered continuously to obtain emission directed primarily according to a predetermined pattern.

In order to allow the sequential triggering of the transducers, the device comprises, for example at a defined depth, a seismic receiver coupled to the formation around the well, which is connected to an acquisition and processing unit suitable for the sequential control of the transducers.

It may also comprise a seismic receiver associated with a plurality of transducers (for example, they are fastened to a bracket fixed to a fastening rod) and connected to an acquisition and processing unit adapted to determine the The travel time of waves between locations, and sequentially controls them.

The moving elements can be of any type: electromechanical, electromagnetic, hydraulic, etc. types. According to a preferred embodiment, each vibrator includes a sensing element post coated with a protective sheath, the coupling material being in contact with the protective sheath and with two end slabs on at least a portion of each of its respective surfaces. The space between the sleeve and the sensitive element can be filled with a fluid such as oil.

The method according to the invention is capable of generating a vibration signal in a subterranean formation according to a directional emission pattern. It includes:

- Assembling several vibrators in the same well, each of these vibrators comprising two slabs, at least one moving element adapted to generate vibrations and transmit them to the plates, and a generator for applying a periodic control signal to the moving elements , each vibrator is disposed in a well or cavity and embedded in at least one solid material that provides its coupling to the subterranean formation, the material and at least a portion of the upper portion of each face corresponding thereon the slabs are in contact at both ends, and

- Sequential control of multiple transducers by means of a control box with time delays between respective triggering times which depend on the spacing between the positions of the transducers and the propagation speed of waves in the formation around the well.

The sequential control of the oscillators includes, for example, applying the oscillator control signal at a fixed frequency f, the phase Φ _i of the fixed frequency is related to the frequency f and to the time delay through the relationship Φ _I =2π.ft _i .

It is also possible to sequentially control the oscillators by applying different fixed-frequency control signals to them to separate them.

According to one embodiment, the method comprises coupling to the rock formation surrounding the vibratory receiver and prioritizing the travel time of waves between each vibrator and said receiver, respectively.

According to another embodiment, the method comprises adding an oscillator receiver connected to the signal acquisition and processing unit and sequentially triggering a plurality of oscillators with time delays between respective trigger times, the trigger times being generated by the plurality of receivers by associating Signals are calculated by means of said units.

Brief description of the drawings

Other features and advantages of the device and method according to the invention will become apparent from the following description of an embodiment given by way of non-limiting example with reference to the accompanying drawings, in which:

- Figure 1 diagrammatically shows oscillators buried in a coupling material such as cement or similar,

- Figure 1a shows the end plate of each vibrator with fastening rods distributed on its periphery,

— Figure 2a shows graphically the coupling modes for each vibrator, where the end-face slabs are coupled to the surrounding rock formations individually via a single-face coupling material,

- Figures 2b and 2c respectively show the cavities provided in the wells at the level of each slab and the specific pattern that allows each cavity to be produced,

- Fig. 3 diagrammatically shows the layout of several vibrators buried at different depths in the well, where the several vibrators are connected to the surface control system, and taking into account the true velocity of the waves in the formation around the well Allows the use of time-delay sequential control of the oscillator,

- Figure 4 diagrammatically shows a vibrator with associated geophones, which allows sequential control of another mode of the vibrator in the well, and

- Figure 5 shows diagrammatically an embodiment of the apparatus in which each slab comprises two plates arranged parallel to each other.

A detailed description

The device according to the invention comprises at least one (and optimally more) vibrator V. The vibrator can be of any type such as electromechanical, electromagnetic, hydraulic, etc.

In the following description, we shall illustrate by way of example the case of a vibrator comprising at least one column 1 of a sensitive element (piezoelectric or magnetically controlled), which is only connected at each of its opposite ends to a thick Tablets 2 and 3 are associated. The posts of the sensitive elements are located centrally with respect to the slabs 2 , 3 and are covered with a deformable membrane 4 . A connecting cable 5 connects the column 1 to a control signal generator 6 .

The vibrator V is arranged in a cavity or well W and a coupling material 7 such as cement or concrete is injected into the well, for example, so as to be in intimate contact with the column 1 over its entire length and also with the opposite sides of each slab 2,3. surface in close contact. In order to allow the coupling material 7 to be evenly distributed in the space between the slabs, the rear one of the slabs may be provided with perforations 8 . The diameter of the slabs 2, 3 must adequately conform to the diameter of the cavity or well W to obtain maximum coupling surface area.

To further improve the coupling and to distribute the pressure in the large volume of coupling material 7 fastening rods 9 of appropriate length can be fixed to the periphery of the slabs 2 , 3 .

According to the embodiment of FIG. 5 , as shown in the figure, each slab 2 , 3 comprises at least two plates 2 a , 2 b arranged parallel to each other and connected by fastening rods 9 . In order to improve the coupling with the coupling material 7, the outer surface of each plate 2a, 2b and coupling rod 9 preferably has non-uniform undulations, such as grooves. As shown, a liquid L such as oil can be filled in the space between the deformable tubular membrane 4 and the post 1 of the sensitive element.

The cement used for coupling must dry without shrinking to ensure a good coupling.

Rather than burying the transducers entirely in a single volume 7 of coupling material, as depicted in Figure 2a, it is also possible to couple each slab separately to the surrounding formation individually through two volumes 7a, 7b of the material. In order to isolate the volumes 7a, 7b from one another, another material 10 is poured between them. Bentonite or a similar material, which has swelling properties once wet, can be used to fill the intervening space. This solution is beneficial, for example, when the mechanical properties of the coupling material differ from those of the surrounding rock formation.

As shown in Figure 2b, if a cavity 11 is provided at the level of each slab on the well wall, according to the variation, the coupling of the vibrator to the formation can be improved. To produce such chambers, suitable drills or explosives can be used, for example. One solution is to lower the drilling tool or explosives into the well, for example (Fig. 2c) at the deployment point where the vibrator is buried, the rod 12 carries the coil 13 of the fuse, the two coils are effectively separated, and was detonated.

According to the embodiment of FIG. 3 , the device comprises several vibrators V1 , V2 , . The oscillator is also buried in one or more coupling materials 7, 10 (Fig. 1 or Fig. 2a).

With this arrangement, a directional effect can be obtained by connecting a plurality of vibrators V1 to Vn to the generator 6 by controlling the box 14 and sequentially actuating the vibrators with a selected time delay.

Vibration signals that are properly emitted downwards are thus amplified, causing damage to those propagating in other directions. The amplitude of the first multiple reflection at the formation surface is thus significantly reduced, the instability of which amplitude is detrimental to the repeatability of the signal.

This trigger time is determined, for example, in the following manner.

Seismic receivers R (hydrophones, geophones, or preferably a combination of these two pickups) are positioned substantially perpendicular to the well containing the elements or at a sufficiently small The propagation time between this receiver R will not substantially differ from the vertical propagation time. This receiver is positioned in the well housing the vibrator, and it is connected to an acquisition and processing unit 15, for example provided in the surface. If several receivers are located along the well below the vibrator, for example the deepest one will be chosen. Surface-mounted receivers can also be used. First measure the travel time of the wave between each vibrator Vi and this receiver R The time delay t _i (i=1 to n) to be applied to different oscillators Vi can be specifically expressed by the relationship Derived from these propagation times, where K is a constant, and ε is +1 or -1 depending on whether the receiver R is configured either above or below the group of elements. The unit 15 controls the application of these time delays to the vibrator by way of the control box 14 .

As described in the applicant's patent application FR-0001792, in applications where the oscillator emits each single frequency, the time delay appears in the form of a frequency phase shift relative to the preceding time through the relation ΦI = 2πfti .

According to the embodiment of FIG. 4 , controlled sequential triggering of transducers placed in the well is possible by associating each seismic receiver, such as geophone G1 . Each geophone is fastened, for example, to a bracket 16 fitted between two fastening rods 9 . The geophones are each connected to an acquisition and processing unit 15 outside the well. In order to adjust the firing delay of any vibrator Vi in real time relative to the first in the sequence, the effective propagation time of a wave between them can be measured by any method that measures the delay between signals, either in the time domain or in the frequency domain , notably measured by cross-correlation between the signals generated by the geophones and the triggering of vibrator Vi taking this effective travel time into account. This delay measurement can be done by cross-correlation. The time delays calculated by the processing unit 15 are passed to the control box 14, which delays each oscillator accordingly relative to the first.

A vibrator comprising a single central post 1 has been described. However, multiple columns of piezoelectric sensitive elements may be inserted between the two slabs 2, 3 without departing from the spirit of the invention.

Claims (22)

1. A device for transmitting waves in an underground rock formation, comprising: at least one vibrator comprising two slabs (2, 3); at least one moving element (1) adapted to generate vibrations and adapted to transmitting vibrations to the slab; and a generator (6) for applying a periodic control signal to the moving element, characterized in that the vibrator is located in a well or cavity (W) and buried in a subterranean formation that provides At least one solid material (7, 10) coupled thereto is in contact with the two end panels (2, 3) on at least a portion of the upper portion on each of their respective faces. 2. A device as claimed in claim 1, characterized in that it comprises a fastening rod (9) associated with at least one slab (2, 3) to increase the mass (7, 10) of vibrator and coupling material ) coupling. 3. An apparatus as in claim 1, characterized in that each slab comprises at least two plates (2a, 2b) arranged at a distance from each other and connected by fastening bars (9). 4. A device as in claim 2 or 3, characterized in that the outer surface of each slab is provided with non-uniform undulations, such as grooves, to increase the coupling area between the device and the coupling material (7, 10) . 5. A device as claimed in any one of claims 2 to 4, characterized in that the fastening rods are provided with non-uniform undulations to increase the coupling area between the device and the coupling material (7, 10). 6. A device as in claim 1 or 2, characterized in that the slab (2,3) is perforated to facilitate the infiltration of coupling material in the space accommodated between the two end panels (2,3). 7. Apparatus according to any one of the preceding claims, characterized in that it includes, at least on opposite end faces thereof, a single-sided solid coupling material distributed thereby to provide coupling between the vibrator and the formation. 8. A device as in claim 7, characterized in that it comprises at least two coupling materials, the first material (7a, 7b) being distributed on two separate pieces on its opposite end faces to provide coupling of the vibrator to the rock formation , and a second material (10) is inserted between the two blocks. 9. A device as claimed in any one of the preceding claims, characterized in that it comprises several vibrators (V1,...,Vn) connected to the signal generator (6), arranged at intervals along the well (W) And both are buried within at least one coupling material (7, 10). 10. A device as in claim 9, characterized in that it comprises a control box (14) interposed between the vibrator (V1, ..., Vn) and the signal generator (6) to allow them to be triggered continuously. 11. Apparatus as in claim 9 or 10, characterized in that it comprises a seismic receiver (R) coupled at a defined depth to the formation around the well, and the seismic receiver is connected to the acquisition of oscillators adapted to sequentially control is connected to a processing unit (15) to obtain directed emission mainly according to a predetermined pattern. 12. A device as in claim 9 or 10, characterized in that it comprises a seismic receiver (G1) associated with a plurality of vibrators (V1) and adapted to determine the waves between the positions of the different vibrators The propagation times and the acquisition and processing unit (15) adapted to sequentially control them are connected to obtain directional emission mainly according to a predetermined pattern. 13. An apparatus as in claims 2 and 12, characterized in that the receiver (G1) is fastened to a bracket (16) fixed to the fastening rod (9). 14. A device as claimed in any one of the preceding claims, characterized in that each vibrator comprises a post (1) of the sensitive element coated with a protective sheath (4), said coupling material being in contact with the protective sheath (4) and with The upper two slabs (2, 3) are in contact on at least a portion of each respective surface thereof. 15. Device according to claim 14, characterized in that the space between the sleeve and the column of the sensitive element is filled with a fluid such as oil. 16. The device as claimed in claim 14, characterized in that the column (1) consists of piezoelectrically or magnetically controlled sensitive elements. 17. Apparatus as claimed in any one of claims 1 to 13, characterized in that each moving element is of the electromechanical, electromagnetic or hydraulic type. 18. A method of generating a vibration signal in a subterranean formation based on a directional emission pattern, characterized in that it comprises: - Assembling in the same well (W) several vibrators (V1, ..., Vn), each vibrator comprising: two slabs (2, 3); at least one moving element (1) suitable for generating vibrations and connecting them to the plate; and a generator (6) for applying periodic control signals to the moving elements, each vibrator positioned in a well or cavity (W) and buried in at least one of the subterranean formations coupled a solid material (7, 10) in contact with two slabs (2, 3) over at least a portion of each of its respective surfaces, and - sequentially control the individual vibrators (V1,...,Vn) by the control box (10) to obtain a directional emission with a time delay between the respective triggering times, which depend on the spacing between the positions of the vibrators and The speed of wave propagation in the rock formation around the well. 19. A method as claimed in claim 18, characterized in that the sequential control of the oscillators comprises applying an oscillator control signal at a fixed frequency f, the phase Φ _i of the fixed frequency being related to said frequency f by the relation Φ _i = 2π.ft _i and related to the delay. 20. A method as claimed in claim 18, characterized in that the sequential control of the transducers comprises the application of various transducer control signals of different fixed frequencies so as to be able to separate them. 21. A method as claimed in any one of claims 18 to 20, characterized in that it comprises coupling to the formation surrounding the seismic receiver (R) and, after separately determining each transducer and said receiver (R) before the propagation time of the wave in between. 22. A method as in any one of claims 18 to 20, characterized in that it comprises adding an oscillator receiver (R, G1) connected to the signal acquisition and processing unit (15), and using The multiple oscillators are sequentially triggered by the time delay between them, and the trigger time is calculated by the unit (15) by calculating the time delay between the signals generated by the multiple receivers.

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